1. Field of the Invention
This invention relates to an image processing apparatus and method, and more particularly, to an image processing apparatus and method for encoding color-image data in order to record it on a recording medium.
2. Description of the Related Art
Recently, it has become easy to store high-definition color-image data due to an increase in the capacity of a storage device. International standardization of encoding methods for various kinds of images has also progressed. So-called JBIG (Joint Bi-level Image Experts Group) method and JPEG (Joint Photographic Experts Group) method are encoding methods which have recently been standardized in the ISO (International Organization for Standardization).
The JBIG method is an encoding method in which information of binary image data is perfectly preserved using a method called a dynamic arithmetic encoding method. Hence, image compression can be performed without degrading characters, line drawings and the like. However, this method is not suitable for compression of data of a gradation image, such as a color natural image or the like because this is a method for binary image data. On the other hand, the JPEG method is an irreversible encoding method for a color natural image. Hence, although degradation is less pronounced even if data of a gradation image, such as a photograph or the like, is encoded with a high compression ratio, peculiar degradation called mosquito noise occurs around characters and fine lines when encoding data thereof. That is, the standardization of JBIG and JPEG has been done assuming that data of a document is encoded according to the JBIG method, and data of a color natural image is encoded according to the JPEG method.
There exists a color image called a color document which includes a color photograph and a document. The assignee of the present application has proposed, in U.S. patent application Ser. No. 357,380 (filed Dec. 16, 1994), a method as shown in FIG. 9 as a method for encoding data of such a color document. In this proposal, character-image regions and color-gradation-image regions of a color document, whose data is to be encoded, are separated from each other. A color image in the character-image region is subjected to JBIG encoding by being converted into binary values comprising black and white, and the color-gradation-image region is subjected to JPEG encoding.
A second method for encoding data of a color document may be considered in which data of a character image is stored by being converted into character codes using an OCR (optical character recognition) technique.
The above-described conventional approaches, however, have the following problems.
That is, a processing time is required for separating regions of a color document. When storing data of a color document in a storage device by encoding it for each region, the user must wait for a long time until the processing is terminated. In the method using the OCR technique, processing for recognizing characters is needed, thereby further increasing the processing time.